(1) Field of the Invention
The present invention relates to a technique to record and play back digital works while protecting digital works from unauthorized use.
(2) Description of the Related Art
Generally, digital contents such as music, video, and computer programs are recorded and copied without any degradation. This feature gives rise to a problem as follows. That is, contents that are available on a chargeable basis may be used or copied by a third party without proper authorization. Such unauthorized use violates the copyright of contents. In addition, with the recent widespread of distribution of music contents on DVD (Digital Versatile Disc) or via the Internet, an increasing numbers of digital contents are distributed. With this being the situation, it is strongly desired that the problem stated above be addressed.
One conventional mechanism addressing the problem is Content Protection for Recordable Media (CPRM), which prohibits unauthorized copying of a content recorded on a recording medium. This mechanism is disclosed, for example, in Matsushita Technical Journal, Vol. 48. No. 2, (April, 2002 issue) published by Matsushita Electric Industrial Co., Ltd. on Apr. 18, 2002 (page 7). The following is a brief description of CPRM.
At the time of manufacturing, a unique medium ID is recorded to each recording medium. A medium ID is information that is readable but not possibly altered or erased. To record a content to such a recording medium, it is required to generate a content key using a media ID of that recording medium, and encrypt the content with the content key. The content is then recorded on the recording medium in encrypted form. The encrypted content recorded on the recording medium may be copied to another recording medium. However, the medium ID of the destination recording medium is different from the medium ID of the source recording medium, so that it is impossible to generate a content key that would properly decrypt the encrypted content on the destination recording medium. That is to say, a content copied to another recording medium can not be decrypted, so that unauthorized copying is prevented.
There is disclosed another mechanism to address the problem stated above (see, for example, JP-A-7-297818). This mechanism is to exclusively allow authorized users who belong to a specific user group to freely copy contents for a backup purpose, for example, or to use contents on a terminal other than a receiving terminal. In this way, a content distributor is capable of preventing unauthorized use of contents by users who do not belong to the user group, so that the copyright of contents are protected. The following is a brief description of this mechanism.
First of all, each user who belongs to a group has an IC card storing a group key unique to the group. In order to distribute a content that should be usable exclusively to the group, a content distributor encrypts the content with the group key unique to the group, and distributes the content in encrypted from. Thus, users who do not belong to the group are not able to use the distributed content as those users are without the group key that is required to decrypt the content. In this way, the content distributor manages to prevent unauthorized use of contents by users who do not belong to the user group.
Unfortunately, however, the conventional mechanisms still have following problems. According to the conventional mechanisms, a usage pattern permitted for authorized users is fixed. To be more specific, for example, a content distributor is not able to flexibly set usage patterns so as to (a) prohibit copying of a content A, (b) prohibit a content B from being used by a device outside a predetermined group, while permitting in-group devices to make backup copies, and (c) permit a content C to be used exclusively on a predetermined device. In short, with the conventional mechanisms, a content distributor can not flexibly permit different usage patterns for each content to be distributed or for each user being a distribution target.